In a liquid crystal display element, the classification based on the operation mode of the liquid crystals includes PC (phase change), TN (twisted nematic), STN (super twisted nematic), OCB (optically compensated bend), IPS (in-plane switching), and VA (vertical alignment), etc. The classification based on the drive system of an element includes PM (passive matrix) and AM (active matrix). PM is classified into static and multiplex, etc., and AM is classified into TFT (thin film transistor) and MIM (metal insulator metal), etc. TFT is classified into amorphous silicon, polycrystal silicon, and continuous grain silicon. Polycrystal silicon is classified into the high temperature type and the low temperature type depending on the production process. The classification based on the source of light includes reflection type using natural light, penetration type using backlight and half penetration type using both natural light and backlight.
These elements comprise a liquid crystal composition having appropriate characteristics. It is necessary to improve the general characteristics of the composition in order to obtain the AM element having excellent general characteristics. The relation between the general characteristics of the composition and those of the AM element is summarized in the following Table 1. The general characteristics of the composition will be explained further based on commercially available AM elements. The temperature range of a nematic phase correlates with the temperature range where the element can be used. A preferable upper limit temperature of a nematic phase is 70° C. or more and a preferable lower limit temperature of a nematic phase is −20° C. or less. The viscosity of the composition correlates with the response time of the element. Short response time is preferable for displaying animation on the element. Therefore, a low viscosity of the composition is preferable. A low viscosity at a low temperature is more preferable.
TABLE 1General characteristics of the composition and theAM elementGeneral characteristics ofGeneral characteristics ofNo.the compositionthe AM element1Temperature range of aTemperature range wherenematic phase is wide.the element can be used iswide.2Viscosity is low. 1)Response time is short.3Optical anisotropy isContrast ratio is large.appropriate.4Positive or negativeDriving voltage is low.dielectric anisotropy isPower consumption is low.large.5Specific resistance is high.Voltage holding ratio ishigh.Contrast ratio is high.1) Time of filling the composition into the liquid crystal display(cell?) can be shortened.
Optical anisotropy of the composition correlates with the contrast ratio of the element. The element which has the VA mode, the IPS mode and so on uses electrically controlled birefringence. Then, to maximize the contrast ratio in the element having the VA mode, the IPS mode and so on, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element is designed to have a constant value. An exemplified value is 0.30 to 0.35 μm (VA mode) or 0.20 to 0.30 μm (IPS mode). Cell gap (d) is usually 3 to 6 μm, and consequently optical anisotropy of the composition is mostly in a range of 0.05 to 0.11. A large dielectric anisotropy of the composition contributes to a low driving voltage of the element. Therefore, a large dielectric anisotropy is preferable. A high specific resistance of the composition contributes to a high voltage holding ratio and a high contrast ratio in the element. Therefore, the composition which has a high specific resistance at the primary stage is preferable. The composition which still has a high specific resistance after prolonged use is preferable.
The composition having positive dielectric anisotropy is used in the AM element which has the TN mode and so on. On the other hand, the composition having negative dielectric anisotropy is used in the AM element which has the VA mode and so on. The composition having either positive or negative dielectric anisotropy is used in the AM element which has the IPS mode and so on. The composition having negative dielectric anisotropy comprises a liquid crystal compound which has negative dielectric anisotropy. This compound has a polarity radical like fluorine on the short axis of the molecule. This compound has 2,3-difluoro-1,4-phenylene, for instance. Examples of the composition having negative dielectric anisotropy are described in the following documents.
[patent documents 1] JP9-278698A (1997) (EP0893423A)
[patent documents 2] JP2003-2858A (U.S. Pat. No. 6,544,604B2)
[patent documents 3] JP2004-35698A
[patent documents 4] JP10-251644A (1998) (U.S. Pat. No. 6,248,410B1)
[patent documents 5] EP0474062A (U.S. Pat. No. 5,384,065B1)